Eectrocardiogram (ECG) may provide valuable informations regarding the infarct-related artery (IRA), which may be of guidance in selecting the therapeutic modality. ST segment elevation in inferior leads usually indicates occlusion of right coronary artery, less often left circumflex coronary artery or rarely occlusion of left anterior descending coronary artery may be the cause. We are to determine whether the initial ECG can differentiate the right coronary artery (RCA) or left circumflex artery (LCx) occlusion in acute inferior myocardial infarction (IMI).

We compared retrospectively the ECG recorded within 12 hours from the onset of chest pain with coronary angiographic findings in 85 patients (34% of all 250 patients) having electrocardiographic criteria for IMI.

1) Angiographic characteristics. Of the 85 patients, IRA was RCA in 65 (76%) (38[58%] proximal, 27[42%] distal to first right ventricular branch), and LCx in 20 (24%) (nine[45%] proximal to first obtuse marginal branch or involving a high first marginal branch, eleven[55%] distal obstruction). RCA dominance was more common in RCA occlusion group (100% vs 80%, p=0.001), and LCx dominance in LCx occlusion group (15% vs 0%, p=0.001). No significant difference was noted between two groups regarding vessels diseased, involvement of left anterior descending coronary artery and contralateral artery (RCA or LCx), location of the lesion. 2) Electrocardiographic characteritics. Lateral limb leads (I, aVL): ST segment depression (≥1 mm) was more common in RCA occlusion group (82% vs 45%, p=0.001). Isoelectric ST segment in I was more common in LCx occlusion group(100% vs 15%, p=0.001). Left precordial leads (V5,6): ST segment elevation (≥1 mm) was more common in LCx occlusion group (60% vs 15%, p=0.001). Isoelectric ST segment was more common in RCA occlusion group (57% vs 20%, p=0.004). ST segment depression (≥1 mm) was not different between two groups. Right precordial leads (V1-4): ST segment changes were not different between two groups. Lead I and left precordial leads (V5,6): Isoelectric ST segment in lead I and ST segment elevation (≥1 mm) in V5 or V6 was more common in LCx occlusion group (60% vs 5%, p<0.05, sensitivity 60% specificity 95% positive/negative predictive value 80%/89%, test accuracy 87%). Amplitude of R wave in V1: Amplitude of R wave in V was greater in LCx occlusion group (3.60±1.42 mm vs 2.20±1.42 mm, p<0.05).

The initial electrocardiogram was useful in differentiating LCx occlusion from RCA occlusion in patients with IMI. Absence of ST segment depression in I and aVL, and ST segment elevation in V(5,6), isoelectric ST segment in I, tall R wave in V(1) were significantly more common in LCx occlusion.